Location: Animal Parasitic Diseases
Title: Functional conservation of Toxoplasma gondii virulence genes in its avirulent relative, Hammondia hammondi Authors
Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 1, 2013
Publication Date: April 15, 2013
Citation: Walzer, K.A., Dam, R.A., Herrmann, D.C., Schares, G., Dubey, J.P., Boyle, J.P. 2013. Functional conservation of Toxoplasma gondii virulence genes in its avirulent relative, Hammondia hammondi. Proceedings of the National Academy of Sciences. 110:7446-7451. Interpretive Summary: Toxoplasma gondii is a single-celled parasite of all warm-blooded hosts worldwide. It causes mental retardation and loss of vision in children, and abortion in livestock. Cats are the main reservoir of T. gondii because they are the only hosts that can excrete the resistant stage (oocyst) of the parasite in the feces. Humans become infected by eating under cooked meat from infected animals and food and water contaminated with oocysts. Why some people become sick, whereas most infected with toxoplasmosis remain asymptomatic, is largely unknown. Parasite genetics is thought to be one factor determining the clinical outcome. Hammondia hammondi is the closest relative of T. gondii, but is non pathogenic, and is currently used to study the pathogenesis of toxoplasmosis. The authors report the transfection of two H. hammondi genes from H. hammondi into t. gondii to determine pathogenesis of virulence of these two related parasites. The results will be of interest to biologists, parasitologists, and public health workers.
Technical Abstract: Toxoplasma gondii is a ubiquitous protozoan parasite capable of infecting all warm blooded animals, including humans. Its closest extant relative, Hammondia hammondi, has never been found to infect humans and in contrast to T. gondii is highly attenuated in mice. To better understand the genetic bases for these phenotypic differences we sequenced the genome of a H. hammondi isolate (HhCatGer041) and found the genomic synteny between H. hammondi and T. gondii to be ~95%. We used this genome to determine the H. hammondi primary sequence and functionality of two major T. gondii mouse virulence determinants, ROP5 and ROP18. When we expressed these genes in T. gondii, we found that H. hammondi orthologs of both ROP5 and ROP18 were functional as virulence genes. Similar to T. gondii, the ROP5 locus is expanded in H. hammondi, and two distinct H. hammondi ROP5 paralogs increased the virulence of a T. gondii ROP5 knockout strain. We also identified a 107 base pair promoter region, absent only in Type III T. gondii, which is necessary for ROP18 expression. This indicates that the ROP18 promoter was active in the most recent common ancestor of these two species, and that it was subsequently inactivated in progenitors of the Type III lineage. Overall these data suggest that the virulence differences between these species are not solely due to the functionality of these key virulence factors, and suggests that other effectors or master regulators of transcription may be responsible.